Above is an analemmatic sundial. The idea is to orient the sundial facing south, and then place a vertical pointer on the central figure-8 track, in a position corresponding to the date. The sundial above shows a simulated shadow for 2:15 PM yesterday. It can be seen that the sundial tells the time reasonably well, thanks to the inbuilt adjustment for variation in solar position.

For large-scale analemmatic sundials, like the one below, people can stand on the central figure-8 track and act as a human pointer. A sundial like this is fun to have in the garden.

Here are blank sundials for some Southern Hemisphere cities:

The ShadowsPro software will also generate sundials like these, if anyone is particularly enthusiastic.


The analemma

If you photograph the sun at the same time every day (or every few days), you will find that the sun traces out a path in the sky, called the analemma. György Soponyai, in Budapest (Hungary), did exactly that at 8 AM each morning between 29 January last year and 6 January this year, to produce the wonderful photograph below (click to zoom):

More analemma photographs (by Anthony Ayiomamitis) can be found here. The shape of the analemma results from the fact that (1) the Earth is tilted on its axis by 23.5° and (2) the Earth orbits the sun in an ellipse, rather than a circle. The diagram below shows the calculated analemma for 12 noon at the Royal Observatory, Greenwich (latitude 51.48° N):

The concept of the analemma can also be used in constructing sundials. If an appropriate analemma is placed in the centre of the sundial, a gnomon placed at the right point on the analemma will correctly tell the time with its shadow (except for daylight-saving, of course).

Such sundials are popular in parks, because the viewer can stand on the analemma at a position corresponding to the current date, and his or her shadow will tell the time, without the need for additional time-of-year correction. I photographed the sundial above and below at Mt Stromlo Observatory in June 2012. It can be seen that the time was about 2:20 PM.

Monitoring space debris from Australia

The NASA Orbital Debris Program Office has long worked on monitoring and managing the serious problem of space debris.

Now, a new Australian Collaborative Research Centre (CRC) for Space Environment Management based at Mt Stromlo Observatory will add to this effort. It will bring together the expertise of two Australian universities and two companies, together with US and Japanese partners. The photo below of the Satellite Laser Ranging (SLR) facility at Mt Stromlo is by Ian Sutton.